This invention relates to a process of addition polymerization, especially olefin polymerization, and in particular to a multistage polymerization process effected using a multi-site polymerization catalyst.
The molecular weight distribution (MWD) of a polymer affects the properties of the polymer, in particular its mechanical strength and processability. Mechanical strength to a large extent is determined by the high molecular weight fraction and processability to a large extent is determined by the low molecular weight fraction. The mechanical strength moreover can be manipulated by the inclusion of n-olefin comonomers, with it thus being possible to vary the nature and relative content of the side chains so introduced. This is particularly important for the high molecular weight portion of the broad MWD polymer, e.g. a PE polymer, and thus the comonomer content of the high molecular weight portion may typically be greater than that in the low molecular weight portion which latter may be a homopolymer. Accordingly polymers with a broad or multimodal (e.g. bimodal) MWD find many uses as for example in blow moulding, films, pipes, etc., where a combination of strength and processability is particularly important.
Certain olefin polymerization catalysts are generally less suitable for the single stage preparation of polymers for such uses because the MWD for the polymers they produce is too narrow and as a result the polymer may be difficult to process.
The preparation of broad MWD olefin polymers is described for example in EP-A-310734, EP-A-128045 and NO-923334.
Thus broad MWD olefins can be made in a dual reactor system (e.g. as described in NO-923334) using a variety of transition metal catalysts, e.g. Ziegler catalysts. The broad MWD results in this case from the processing conditions in the different reactors favouring the production of different molecular weight polymers, e.g. one favouring the production of a higher molecular weight polymer and a second favouring production of a lower molecular weight polymer. Broad MWD polyolefins may also be produced in a single reactor using either catalyst mixtures or multisite catalysts, ie. within the same process conditions the different catalysts or different catalytic sites favour production of polymers of different molecular weights. This arises since the different catalytic sites may have significantly different propagation/termination rates for olefin polymerization (see for example EP-A-310734).
In addition to being used in processes with essentially a single reactor, such multisite catalysts may be used in processes with several reactors, for example, where the reactor conditions are so adjusted that polymers with approximately the same characteristics are made in several of these reactors.
We have now found that the MWD of a polyolefin can be particularly effectively tailored to suit the needs of the user of the polyolefin, e.g. the producer of blow moulded objects, cables, tubes and pipes, etc., if polymerization is effected in at least two reaction stages using a catalyst material, generally a particulate material, that contains at least two different types of active polymerization sites. Typically such a catalyst material may contain a particulate multi-site component together with, in a liquid phase, co-catalysts and adjuvants.
Thus viewed from one aspect the invention provides a process for the preparation of an olefin polymer wherein olefin polymerization is effected in a plurality of polymerization stages, optionally in a plurality of polymerization reactors, in the presence of an olefin polymerization catalyst material, characterized in that said catalyst material comprises at least two different types of active polymerization sites.
The reactor used in one stage of the process may be used in a subsequent polymerization stage. Where the process of the invention is effected in a single reactor vessel, polymerization stages will conveniently be effected using different monomer/comonomer mixtures and optionally different process conditions (ie. temperature, pressure, reaction time, etc.).
It is particularly preferred that no one of the reaction stages used in the process of the invention be used to produce more than 95% by weight of the overall polymer, more particularly no more than 90%, especially no more than 85%, more especially no more than 78% and most especially no more than 70%. Thus if a prepolymerization is effected to produce a catalyst-polymer material for use in the process of the invention, that process will generally involve the use of at least two more reaction stages, such stages producing more than 93% by weight, preferably more than 96% by weight, particularly preferably more than 98% by weight of the polymer material. In the absence of prepolymerization, the process of the invention will involve at least two reaction stages capable of producing up to and including 100% by weight of the polymer material. Preferably however, at least 10% by weight of the total polymer should be made in each stage.
Furthermore it is especially preferred that at least two different reactants selected from monomer, comonomer and hydrogen be used in at least two of the reaction stages whereby at least one of the catalytic sites is caused to produce a different polymer in two different reaction stages. In this way, the tailoring of the high molecular weight end of the molecular weight distribution discussed below can be achieved.